The invention relates to laser processing systems, and relates in particular to optical elements for use in high power laser processing systems.
The power output of conventional commercially available lasers may be provided at the multi kilowatt level. Because lasers have low overall power efficiency, they become heated by the input power that is not converted into useful output. At a typical overall efficiency of 10%, very large amounts of heat accumulate in the laser, and this heat is typically removed by means of circulating cooled water, forced air, or a combination thereof. The fixed or semi-fixed optical elements used to bend, focus, and direct the output laser beam also become heated by the waste heat of the laser, but more importantly are irradiated by the laser beam itself. Because these optical elements cannot be either completely transparent or perfectly reflective, they absorb and convert a very small fraction of the laser power into additional heat, which heat must also be removed. The cavity mirrors, folding flats, collimating telescope and the like are generally part of the laser package itself, and so can easily be cooled by whatever means is employed for the laser.
The system used to direct the laser beam onto a work piece however, commonly called a scan head, is often physically remote from the laser itself, and may even be in the form of a robot end-effector or otherwise dynamically connected to the laser itself. This remote and dynamic connection between the laser and the process makes it very difficult to cool the scanning system using the same cooling system built into the laser, and it is typically difficult and expensive to provide an equivalent cooling system at the location of the scanner.
The reason that the scanning system needs to be cooled is as follows. The laser beam irradiates the optics, which typically include one or two mirrors and a focusing lens. Although these optics are outside the hot laser environment and the laser beam is expanded at their location to reduce the power density of the beam, not all of the power impinging on the mirror or mirrors is reflected. A small fraction of the laser beam power, typically 0.3% to 0.5% becomes available to heat each mirror. At a beam power of 6 kW, 0.3% is 30 Watts, which, if absorbed, would quickly heat the mass of the mirror to a temperature that would destroy it.
The mirrors used in the laser itself are fixed in position, and therefore allow robust thermal contact with the frame of the laser, which if actively cooled provides a conduction path of low thermal resistance to the active cooling medium. The mirrors used to direct the laser beam to the work in the other hand, are generally supported on a slender actuator shaft, typically that of a limited-rotation torque motor. For reasons relating to the sensitivity of the motor itself to the influence of heating, the shaft is intentionally made of a material with high thermal resistance, such as stainless steel. As a result, the only effective cooling mechanism for the mirror is natural convection.
It can be shown that the loss of heat by free convection from a flat plate is about 6.6×10−2 W/cm2 of plate surface when the surrounding air is at, for example, 20° C. and the plate is at, for example, 50° C. This puts an upper limit on the power that the plate, a mirror in this case, can absorb if it desired to keep its temperature at or below 50° C. This is typically desired because otherwise the figure of the mirror is likely to change away from the ideal, and also because the performance of the actuator to which it is attached is, in general, degraded by heat.
The conventional process for designing a laser processing mirror has been to choose a beam aperture, and then a mirror size that is large enough to produce a focused spot size appropriate to the job at hand. The minimum beam diameter required is given by D=(1.22λ)(F)/spot diameter where F is the lens focal length and λ is the laser wavelength. For example, at the wavelength of CO2 lasers (10.6 microns or 1.06×10−3 cm), and a focal length of 20 cm, the minimum aperture required to form a 1×10−2 cm diameter spot is (1.22)(1.06×10−3)(20)/(1×10−2)=2.59 cm diameter. The area of a mirror with this aperture, designed to operate with a nominal angle of incidence of 45 degrees, and allowing for some border around the clear aperture, would be about 1.65 D2, or, in this case, a little over 11 cm2, and could dissipate 6.6×10−2 W/cm2×11 cm2=0.73 W at a temperature of 50° C. surrounded by air at 20° C. (A 30 C temperature rise, and an absolute temperature of 50 C) Assuming that the reflective coating reflects 99.7% of the beam, this mirror may be used with about 0.73/0.003=244 Watts of beam power, which until recently was quite adequate. However, as shown previously, with laser beam powers now in the kilowatt range, up to 25 times the allowable heat could be absorbed by such a mirror, so a new, more efficient mirror design is required if we are to avoid the complexity, expense, and system degradation active cooling can cause. We will call such a mirror a low absorption mirror.
There is a need therefore, for an economic and efficient high power laser processing system that provides improved performance of laser processing without relying on active cooling.